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Only if during the time the flap is normally closed, the engine is being held back by the failure of sufficient air to move through the exhaust. This is not the case.

Even a full exhaust replacement makes relatively little difference in HP and torque on this engine, perhaps 3%. Accordingly, a tiny mod which effects only part of the exhaust in during a limited RPM range is doing nothing.

Besides, if power was this easy to generate and the only downside is a slightly louder engine, don't you think someone at BMW would have thought of this? It takes a untrained, backyard mechanic to figure this out?

The perceived change is purely emotional - "it's a tad louder and I made a change. Therefore there must be more power."

Only if during the time the flap is normally closed, the engine is being held back by the failure of sufficient air to move through the exhaust. This is not the case.

Even a full exhaust replacement makes relatively little difference in HP and torque on this engine, perhaps 3%. Accordingly, a tiny mod which effects only part of the exhaust in during a limited RPM range is doing nothing.

Besides, if power was this easy to generate and the only downside is a slightly louder engine, don't you think someone at BMW would have thought of this? It takes a untrained, backyard mechanic to figure this out?

The perceived change is purely emotional - "it's a tad louder and I made a change. Therefore there must be more power."

Power is that easy to generate all other things being equal. Yup, it was shadetree guys who tried & liked it. Popular thought's that noise regs created the flap, tho emission volume [not %] may have played a part.

Hey, try it. If you no like, easy enough t'return to stock. Looking for map specific settings so's can snip the wire tie....

I appreciate that you believe this. Expectation bias is exceedingly powerful.

Engine engineers will sell their first-born for a 1HP increase. They make many minute changes to engine design for much less than this. If removing the flap increased power to this degree it would not be there.

The 3% improvement realized with a full cat-back is difficult to feel as seat of the pants. But, it's louder so the driver thinks he feels more power. Even on a dyno one has to be careful to control all variables (heat soak, etc.) to even accurately measure a difference.

Let's assume the flap-mod produces a full ten percent of a cat-back - a very generous estimate. You honestly believe one can truly and accurately feel a 1-2 HP increase in a 250hp-300hp engine?

Enjoy the mod if you like the sound. Don't fool yourself into thinking you have a perceivable, or even fractionally measurable, increase in power.

Then there is the magic spark plugs, special magnets, EMI/RFI filters, special power producing oil additives . . . in no time an F30 can produce 400hp - 500hp. Easy.